Traffic Engineering Using Longest Match Routing

Consider the topology:

I am going to demonstrate how to effectively use the redundant links between R1 and R2 by manipulating the longest match routing principle.

Remember, longest matched prefix is the first thing that the router looks for when choosing the best route.
It even takes precedence over connected routes. We will see this in the next example.

There are 2 routes that R1 can take to reach R2's loopback, the frame relay link and the serial link.  I want to create a static route so that R1 will use the serial link to reach R2's loopback. Vice-versa.
R1 and R2 static routes:

When R1 does a lookup on the final destination  2.2.2.2, the longest match is 2.2.2.0/24 out via 12.12.12.2 (serial link). For any other destinations, such as 2.2.1.1, the longest match will be 2.2.0.0 via the frame-relay. 

Longest Prefix Match Routing Choosen over Connected Route Example:

This example is to show that longest prefix match routing wins over connected route:
Same as above topology, this time I've added a loopback 1 on R1 (2.0.0.2/8)

In R1's routing table, I now have a connected route (2.0.0.0/8)

When R1 does a lookup on the final destination  2.2.2.2, the longest match is 2.2.2.0/24 out via 12.12.12.2 (serial link).

 

Watch out!!!

If I traceroute to 2.2.1.1, which path will the route choose? Let's try it...

 Remember, router will look for the longest prefix match route, 2.2.1.1 matches 2.2.0.0/16 via 102.102.102.2.  But since we don't have a host for 2.2.1.1, we get "UNREACHABLE"